Non-reciprocal field displacement isolator

ABSTRACT

A rectangular ferrite bar is asymmetrically arranged in a rectangular waveguide and is prepolarised in the direction of the smallest dimension of the waveguide by a static magnetic field. Electrically conductive strips the longitudinal dimension of which coincides with the direction of the magnetic field, are provided on the ferrite bar.

Leenders et al.

May 6, 1975 NON-RECIPROCAL FIELD DISPLACEMENT ISOLATOR Inventors:Johannes Petrus Leenders; Peter Caspar Stephanus Rutjes, both ofEmmasingel, Eindhoven,

Netherlands Assignee: U.S. Philips Corporation, New

York, N.Y.

Filed: Feb. 14, 1974 Appl. No.: 442,471

Foreign Application Priority Data [56] References Cited UNITED STATESPATENTS 3,00L154 9/l96l Reggia 333/73 W 3.082383 3/l963 Stern r. 333/17Primary ExaminerPaul L. Gensler Attorney, Agent, or FirmFrank R.Trifari; George B. Berka [57] ABSTRACT A rectangular ferrite bar isasymmetrically arranged in a rectangular waveguide and is prepolarisedin the direction of the smallest dimension of the waveguide by [973Netherlands "730400[ a static magnetic field. Electrically conductivestrips the longitudinal dimension of which coincides with the LS. Cl- Mdirection of the magnetic field. are provided on the int. Cl. HOlp 1/32ferrite bar Field of Search 333/].1, 24.1, 24.2, 24.3,

333/73 w 3 Claims, 4 Drawing Figures PERMANENT MAGNET 6 3 -4 fm 5FERRITE I I conoucnvs 7 STRIPS PERMANENT MAGNET NON-RECIPROCAL FIELDDISPLACEMENT ISOLATOR The invention relates to a non-reciprocal fielddisplacement isolator. comprising a rectangular waveguide and arectangular bar of gyromagnetic material which is arranged in thewaveguide at a small distance from one of the side walls. the side facesofthe said bar extending parallel to the waveguide walls. the bar of gyromagnetic material being prepolarised in the direction of the smallestdimension of the waveguide by a static magnetic field.

A non-reciprocal isolator is generally used in the microwave field as anisolating member between a source and a load. the part ofthe energyapplied by the source to the load which is reflected by the load beingattenu ated by the non-reciprocal isolator so as to prevent thereflected energy from influencing the source.

A non-reciprocal field displacement isolator as herein described inwhich a homogeneously distributed resistance layer is provided on thesurface of the bar which faces the centre of the waveguide is inter aliaknown from the article The field displacement isolator" by S. Weisbaumand H. Seidel published in "Bell System Technical Journal No. 35 inI956. pages 877-898.

The operation of this field displacement isolator is based on theproperty that a wave of the type H which propagates in the waveguidecontains an electric field component at the area of the resistance layerwhose strength is dependent of the direction of propagation. If the baris suitably proportioned and suitably arranged in the waveguide and ifthe static magnetic field has a given size. the strength is maximum forone of the propagation directions and is minimum (substantially null)for the other propagation direction. As a result. the re sistance layerattenuates the wave which has a maximum electric field strength at thisarea. and allows sub stantially unattenuated passage of the wave whichhas a minimum electric field strength at this area.

This known non-reciprocal field displacement isolator has the drawbackthat it has a rather small isolation per unit of length of the isolator.which necessitates a comparatively long and hence heavy and unpracticalisolator configuration if an isolator having a high isolation isrequired.

Furthermore. the heat is dissipated in the very thin resistance layer.after which it is depleted to the bar which is made. for example. offerrite. This implies that the heat developed per unit oftime and hencethe maxim um permissible power for which the isolator can still be usedis determined by the very thin resistance layer.

These drawbacks are alleviated according to the invention which has forits object to realize a nonreciprocal field displacement isolator whichhas a comparatively high isolation per unit of length of the isolator.

The non-reciprocal field displacement isolator according to theinvention is characterized in that electrically properly conductivestrips which are insulated from each other by intermediate spaces areprovided on the surface of the bar of gyromagnetic material which facesthe centre of the waveguide. the longitudinal dimensions of the saidstrips coinciding with the direction of the static magnetic field.

The invention and its advantages will be described in detail hereinafterwith reference to the embodiment shown in the drawing. correspondingparts being denoted by the same references in the various Figures.

FIG. I is a front view of the non-reciprocal isolator according to theinvention.

FIG. 2 is a sectional view taken according to the line A-A of thenon-reciprocal isolator shown in FIG. 1.

FIG. 3 is a sectional view taken according to the line BB of thenon-reciprocal isolator shown in FIG. 1, and

FIG. 4 shows graphs of the insertion loss and isolation as functions ofthe frequency of the non-reciprocal isolator shown in FIG. I.

The non-reciprocal isolator shown in the FIGS. I. 2 and 3 comprises arectangular waveguide l which is provided with connection flanges 2 and3 and a rectangular bar 5 of ferromagnetic material such as ferritewhich is mounted in the waveguide by way of a dielectric holder 4. Thisferrite bar 5 is arranged in the waveguide such that its side facesextend parallel to the walls of the waveguide and that it is situated ata small dis tance from one of the side walls. Also provided are permanent magnets 6 and 7 which are interconnected by a magnetic yoke whichis situated outside the wave guide and which is not shown in theFigures. By means of these magnets. the ferrite bar 5 has beenprepolarised to a point far below the gyromagnetic resonance by aninternal static magnetic field H which extends parallel to the sidewalls of the waveguide. For a given proportioning of the ferrite bar 5.a given loca tion in the width ofthe waveguide and a given direction andstrength of the static magnetic field H... it is achieved in knownmanner that in a wave of the type Ho! which propagates from flange 2 inthe direction of flange 3. to be referred to hereinafter as the positivedirection. a minimum strength of the electric field component occurs atthe area of the surface of the ferrite bar 5 which faces the centre ofthe waveguide. to be referred to hereinafter as the active surface.Similarly. for a wave of the type H which propagates in the negativedirection it is achieved that at the area of the active surface amaximum strength of the electric field component occurs.

In known non-reciprocal field displacement isolators a thin layer ofgraphite emulsion or a very thin homogeneous metal layer is deposited atthe area of the active surface by spraying. spreading. or vapourdeposition. with or without the assistance of a dielectric foil which isused as a carrier. In this layer power is dissipated by a wave whosemaximum electric field component oc curs at the area of the activesurface. with the result that this wave is attenuated. and a wave of thetype H which propagates in the opposite direction is allowed to passsubstantially unattenuated. For an attenuation layer having a layerresistance of to Ohms/- square. an attenuation of maximum 5 dB per cm oflength of the non-reciprocal isolator is obtained at 10 GHz.

So as to achieve a substantial increase of the isolation per length unitof the isolator. according to the invention electrically properlyconductive strips 8 which are insulated from each other by intermediatespaces and whose longitudinal dimensions coincide with the direction ofthe static magnetic field H,, are provided at the area of the activesurface of the ferrite bar 5.

The operation of the non-reciprocal isolator is as follows.

As already stated. a wave of the type H which propagates in the negativedirection has an electric field component of maximum strength at thearea of the ac tive surface of the ferrite bar 5. the direction of thiscomponent coinciding with the longitudinal direction of the strips.Under the influence thereof. large cur' rents flow in the properlyconductive strips 8. These large currents are accompanied by strongmagnetic fields which are situated in planes perpendicular to thedirection of these currents. Half of the lines of force of these fieldsextend outside the ferrite bar 5 and the other half extends inside theferrite bar 5. the lines being closed via the intermediate spaces ofadequate dimensions which are provided between the strips. with theresult that very strong fields arise. In the ferrite bar these fieldsextend perpendicular to the inner static magnetic field H... Thecircularly polarised component of this field. whose rotary directioncorresponds to the rotary direction of the spins in the ferrite barwhich is determined by the direction of the static magnetic field H willtransfer its energy to the spins in known manner. As a result. the wavepropagating in the negative direction will be substantially attenuated.

The isolation of such a non-reciprocal isolator amounts to at least dBper cm of length of this isola tor. so that a short and hence lightnonreciprocal isola tor has been realized fora given required isolation.The injection loss of a wave of the type H propagating in the positivedirection amounts to only a few tenths of dB. P16. 4 shows the isolation01 and the injection loss a of such a non-reciprocal isolator.comprising a ferrite bar having a length of 0.9 cm. as a function of thefrequency.

In this type of non-reciprocal isolator the heat is developed directlyin the ferrite body. with the result that. moreover. this isolator isbetter suitable for use with higher powers than the known nonweciprocalisolator comprising a resistance layer.

In order to achieve an optimum effect. the strips 8 must contact theferrite bar 5 without intermediate layer such as an adhesive layer. Tothis end. a foil 9 of. for example. Mylar is used on which the stripsare glued or vapour-deposited. after which the side ofthe foil on whichthe strips are provided is arranged against the active ferrite surfaceand is secured such that the strips are in direct contact with theferrite.

So as to achieve an optimum effect it is also neces sary that the strips8 are as long as possible. However, they may not be substantially longerthan one half of the wavelength of the HF oscillation which occurs inthe ferrite at the operating frequency. whilst the distance between thestrips amounts to approximately one quarter of this wavelength. However.it is to be noted that the strips need not be uniform. nor need thespacing be equal. even though this results in an optimum effect for onegiven frequency.

The finite thickness of the strips and the section of the ferrite barlocally cause impedance variations which cause reflections. So as tocounteract this phenomenon as much as possible. a U-shaped matchingelement ll is provided as shown in the FIGS. 1 and 2. This element 11and the foil 9 are omitted in FIG. 2 so as to allow a clear view of thearrangement of the conductive strips 8. The matching element ll is made.for example. of Alundum. and is arranged with its back against the foil9. the legs of the U being perpendicular to the active ferrite surface.A reflection coefficient of less than 1.24 is thus realized over theentire band from 9.1 to 9.6 GHZ.

Due to the use of the ferrite bar 5 and the matching element ll.undesired modes can locally propagate. So as to counteract theappearance of these modes. an inductive element 12 is provided halfwaythe ferrite bar 5, the said element being formed by a partition 12 whichextends from the wave guide side wall opposite to the active surface ofthe ferrite bar 5 over the entire waveguide height almost as far as thecentre of the waveguide. The impedance of this inductive element 12 hasbeen taken into account in the proportioning of the matching element 11.

What is claimed is'.

l. A non-reciprocal field displacement isolator, comprising arectangular waveguide and a rectangular bar of gyromagnetic materialwhich is arranged in the waveguide at a small distance from one of theside walls. the side faces of the said bar extending parallel to thewaveguide walls. the bar of gyromagnetic material being prepolarised inthe direction of the smallest dimension of the waveguide by a staticmagnetic field, characterized in that electrically properly conductivestrips which are insulated from each other by intermediate spaces areprovided on the surface of the bar of gyromagnetic material which facesthe centre of the waveguide. the longitudinal dimensions of the saidstrips coinciding with the direction of the static mag netic field.

2. A non-reciprocal field displacement isolator as claimed in claim 1,characterized in that the strips are in direct contact with the bar ofgyromagnetic material.

3. A non-reciprocal field displacement isolator as claimed in claim 1,characterized in that. so as to suppress undesired modes. an inductiveelement is pro vided between the surface of the bar of gyromagneticmaterial which faces the centre of the waveguide and the oppositelysituated part of the waveguide side wall. l= i i i

1. A non-reciprocal field displacement isolator, comprising arectangular waveguide and a rectangular bar of gyromagnetic materialwhich is arranged in the waveguide at a small distance from one of theside walls, the side faces of the said bar extending parallel to thewaveguide walls, the bar of gyromagnetic material being prepolarised inthe direction of the smallest dimension of the waveguide by a staticmagnetic field, characterized in that electrically properly conductivestrips which are insulated from each other by intermediate spaces areprovided on the surface of the bar of gyromagnetic material which facesthe centre of the waveguide, the longitudinal dimensions of the saidstrips coinciding with the direction of the static magnetic field.
 2. Anon-reciprocal field displacement isolator as claimed in claim 1,characterized in that the strips are in direct contact with the bar ofgyromagnetic material.
 3. A non-reciprocal field displacement isolatoras claimed in claim 1, characterized in that, so as to suppressundesired modes, an inductive element is provided between the surface ofthe bar of gyromagnetic material which faces the centre of the waveguideand the oppositely situated part of the waveguide side wall.